=head1 NAME
-perlfork - Perl's fork() emulation (EXPERIMENTAL, subject to change)
+perlfork - Perl's fork() emulation
=head1 SYNOPSIS
- WARNING: As of the 5.6.1 release, the fork() emulation continues
- to be an experimental feature. Use in production applications is
- not recommended. See the "BUGS" and "CAVEATS AND LIMITATIONS"
- sections below.
+ NOTE: As of the 5.8.0 release, fork() emulation has considerably
+ matured. However, there are still a few known bugs and differences
+ from real fork() that might affect you. See the "BUGS" and
+ "CAVEATS AND LIMITATIONS" sections below.
Perl provides a fork() keyword that corresponds to the Unix system call
of the same name. On most Unix-like platforms where the fork() system
To the Perl program that called fork(), all this is designed to be
transparent. The parent returns from the fork() with a pseudo-process
-ID that can be subsequently used in any process manipulation functions;
+ID that can be subsequently used in any process-manipulation functions;
the child returns from the fork() with a value of C<0> to signify that
it is the child pseudo-process.
=item kill()
-kill() can be used to terminate a pseudo-process by passing it the ID returned
-by fork(). This should not be used except under dire circumstances, because
-the operating system may not guarantee integrity of the process resources
-when a running thread is terminated. Note that using kill() on a
-pseudo-process() may typically cause memory leaks, because the thread that
-implements the pseudo-process does not get a chance to clean up its resources.
+C<kill('KILL', ...)> can be used to terminate a pseudo-process by
+passing it the ID returned by fork(). The outcome of kill on a pseudo-process
+is unpredictable and it should not be used except
+under dire circumstances, because the operating system may not
+guarantee integrity of the process resources when a running thread is
+terminated. The process which implements the pseudo-processes can be blocked
+and the Perl interpreter hangs. Note that using C<kill('KILL', ...)> on a
+pseudo-process() may typically cause memory leaks, because the thread
+that implements the pseudo-process does not get a chance to clean up
+its resources.
+
+C<kill('TERM', ...)> can also be used on pseudo-processes, but the
+signal will not be delivered while the pseudo-process is blocked by a
+system call, e.g. waiting for a socket to connect, or trying to read
+from a socket with no data available. Starting in Perl 5.14 the
+parent process will not wait for children to exit once they have been
+signalled with C<kill('TERM', ...)> to avoid deadlock during process
+exit. You will have to explicitly call waitpid() to make sure the
+child has time to clean-up itself, but you are then also responsible
+that the child is not blocking on I/O either.
=item exec()
waiting pseudo-process that called exec(), not the real process it is
waiting for after the exec().
+When exec() is called inside a pseudo-process then DESTROY methods and
+END blocks will still be called after the external process returns.
+
=item exit()
exit() always exits just the executing pseudo-process, after automatically
wait()-ing for any outstanding child pseudo-processes. Note that this means
that the process as a whole will not exit unless all running pseudo-processes
-have exited.
+have exited. See below for some limitations with open filehandles.
=item Open handles to files, directories and network sockets
pseudo-child created by it that is also a pseudo-parent will only exit
after their pseudo-children have exited.
-A way to mark a pseudo-processes as running detached from their parent (so
-that the parent would not have to wait() for them if it doesn't want to)
-will be provided in future.
+Starting with Perl 5.14 a parent will not wait() automatically
+for any child that has been signalled with C<sig('TERM', ...)>
+to avoid a deadlock in case the child is blocking on I/O and
+never receives the signal.
-=head2 CAVEATS AND LIMITATIONS
+=head1 CAVEATS AND LIMITATIONS
=over 8
One can avoid this by opening files that need distinct seek pointers
separately in the child.
+On some operating systems, notably Solaris and Unixware, calling C<exit()>
+from a child process will flush and close open filehandles in the parent,
+thereby corrupting the filehandles. On these systems, calling C<_exit()>
+is suggested instead. C<_exit()> is available in Perl through the
+C<POSIX> module. Please consult your system's manpages for more information
+on this.
+
+=item Open directory handles
+
+Perl will completely read from all open directory handles until they
+reach the end of the stream. It will then seekdir() back to the
+original location and all future readdir() requests will be fulfilled
+from the cache buffer. That means that neither the directory handle held
+by the parent process nor the one held by the child process will see
+any changes made to the directory after the fork() call.
+
+Note that rewinddir() has a similar limitation on Windows and will not
+force readdir() to read the directory again either. Only a newly
+opened directory handle will reflect changes to the directory.
+
=item Forking pipe open() not yet implemented
The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
else {
# child
while (<STDIN>) { print; }
- close STDIN;
exit(0);
}
else {
# child
print "pipe_from_fork\n";
- close STDOUT;
exit(0);
}
=back
-=head1 BUGS
+=head1 PORTABILITY CAVEATS
-=over 8
+In portable Perl code, C<kill(9, $child)> must not be used on forked processes.
+Killing a forked process is unsafe and has unpredictable results.
+See L</kill()>, above.
-=item *
+=head1 BUGS
-Perl's regular expression engine currently does not play very nicely
-with the fork() emulation. There are known race conditions arising
-from the regular expression engine modifying state carried in the opcode
-tree at run time (the fork() emulation relies on the opcode tree being
-immutable). This typically happens when the regex contains paren groups
-or variables interpolated within it that force a run time recompilation
-of the regex. Due to this major bug, the fork() emulation is not
-recommended for use in production applications at this time.
+=over 8
=item *
=item *
+In certain cases, the OS-level handles created by the pipe(), socket(),
+and accept() operators are apparently not duplicated accurately in
+pseudo-processes. This only happens in some situations, but where it
+does happen, it may result in deadlocks between the read and write ends
+of pipe handles, or inability to send or receive data across socket
+handles.
+
+=item *
+
This document may be incomplete in some respects.
=back
https://perl5.git.perl.org / perl5.git / blobdiff